Abstract Purpose: Molecular-based cancer tests have been developed to augment the standard clinical and pathologic features used to tailor treatments to individual breast cancer patients. Homologous recombination (HR) repairs double-stranded DNA breaks and promotes tolerance to lesions that disrupt DNA replication. Recombination Proficiency Score (RPS) quantifies HR efficiency based on the expression of four genes involved in DNA damage repair. We hypothesized low RPS values can identify HR-deficient breast cancers most sensitive to DNA-damaging chemotherapy. Experimental Design: We collected pathologic tumor responses and tumor gene expression values for breast cancer patients that were prospectively enrolled on clinical trials involving preoperative chemotherapy followed by surgery (N = 513). We developed an algorithm to calculate breast cancer–specific RPS (RPSb) values on an individual sample basis. Results: Low RPSb tumors are approximately twice as likely to exhibit a complete pathologic response or minimal residual disease to preoperative anthracycline-based chemotherapy as compared with high RPSb tumors. Basal, HER2-enriched, and luminal B breast cancer subtypes exhibit low RPSb values. In addition, RPSb predicts treatment responsiveness after controlling for clinical and pathologic features, as well as intrinsic breast subtype. Conclusions: Overall, our findings indicate that low RPS breast cancers exhibit aggressive features at baseline, but they have heightened sensitivity to DNA-damaging chemotherapy. Low RPSb values in basal, HER2-enriched, and luminal B subtypes provide a mechanistic explanation for their clinical behaviors and genomic instability. RPSb augments standard clinical and pathologic features used to tailor treatments, thereby enabling more personalized treatment strategies for individual breast cancer patients. Clin Cancer Res; 23(15); 4493–500. ©2017 AACR.